How is mileage calucated?

I thight I understood this but having just checked my recent VS5/6 flight I do not understnad why the 50% miles flown bonus is different for each direction.

Does this make sense?

Base mileage 4430 miles

Virgin Atlantic, LHR-MIA, Premium Economy 6,645 3
Gold Membership Offer. 50% Miles Flown Bonus 2,259 0
Total 8904 miles

Virgin Atlantic, MIA-LHR Upper Class 8,860 5
Gold Membership Offer. 50% Miles Flown Bonus 2,215 0
Total 11075 miles

Also I had not realsied that the difference in miles earned between J and W is not that great.

[mcmbenjamin]

The bonus has been odd for sometime now. Normally you ead up with more than the 50 percent bonus, like you did on the LHR-MIA flight.

[preiffer]

Looks like a plain old screw-up in your favour (of 44 whole miles [ii]).

Wouldn't worry about it - they're often *slightly* out, usually above, too.